With regard to communication in a network, various techniques have been developed from various viewpoints, such as control in case of occurrence of a fault, redundancy for fault tolerance, and routing control.
For instance, in order to facilitate reduction in network cost and quickly determine a fault, the following communication system has been disclosed. That is, a transmitter-side router A continuously transmits, to a receiver-side router B, packets including user packets at time intervals shorter than a fault determination time that is agreed to by the router B in advance. The router B includes a timer expiring in the fault determination time.
In a case where a network between the routers A and B is normal, the router B receives a packet before expiration of the timer. By using this, the router B judges that the network is normal and resets the timer if it receives a packet before expiration of the timer. If the timer expires before reception of a packet, the router B can judge that the network is not normal and therefore determines that a fault occurs. Such application of the determination by the timer to transmission and reception of packets including user packets enables quick determination of a fault in the network, while avoiding network duplication and while facilitating reduction in cost.
The following telephony system has also been disclosed in order to solve a problem that a conventional Internet telephony system is incapable of distinguishing whether voice packets are not received due to silence compression or the voice packets cannot be received owing to occurrence of abnormality in the Internet.
That is, after a call state has been established between telephones, a calling-side media gateway periodically transmits a monitoring packet to a called-side media gateway via the Internet. When receiving the monitoring packet, the called-side media gateway transmits a response packet to the calling-side media gateway via the Internet. When the number of times at which response or monitoring packets to be received fail to be periodically inputted reaches a prescribed threshold, the two media gateways issue a non-packet-receipt notification to a call agent. When receiving the non-packet-receipt notification, the call agent controls the two media gateways to execute a disconnection process.
The following communication path switching control system has also been disclosed in order to solve a problem that, even if a communication path is switched by a router, apparatuses other than the router cannot recognize that the communication path has been switched.
The communication path switching control system includes a first IPGW (Internet Protocol GateWay) for accommodating and connecting a first PBX (Private Branch eXchange), and a first router responsible for a communication interface between a network and the first IPGW. When detecting a line fault in a communication path currently in use among a plurality of communication paths in the network, the first router searches for a communication path for bypassing the line fault from among the plurality of communication paths, and switches its connection to the searched-for communication path. The first router includes a switching information notification section that notifies, upon switching of the connection to the searched-for communication path, the first IPGW of path switching information pertaining to the communication path to which the connection has been switched. The first IPGW includes a line control section that performs, when detecting the path switching information from the switching information notification section, a line control on the first PBX according to the path switching information.
Incidentally, in recent years, ad hoc networks have become a focus of attention. As to optimal routing in an ad hoc wireless communication network, the following system and method have been disclosed for calculating an optimal route at a node.
That is, routing metric used in the system and the method, if carefully chosen, can provide stability to a network and also provide features like self-healing and load balancing. A routing metric is calculated as a scalar number based upon a number of factors, such as the number of hops, a data rate, link quality, and a device type. Each factor can be determined by evaluation of hallo messages, or other routing messages as required.
Not only wireless ad hoc networks but also wired ad hoc networks have been researched, and application thereof to a sensor network has also been attempted.
For instance, in a wired sensor network referred to as “S-wire”, each node apparatus is connected to a plurality of node apparatuses in a wired manner, and data communication and power supply are performed in a wired manner. Advantages of the wired system include a feature that the sensors can be embedded in earth, water, a structure and the like and a feature that a breakage and the like can be detected.
Some documents, such as those in the following list, are known.    Japanese Laid-Open Patent Publication No. 2003-273964    Japanese Laid-Open Patent Publication No. 2002-271399    Japanese Laid-Open Patent Publication No. 2006-340165    Japanese Laid-Open Patent Publication No. 2006-526937    Tadashige Iwao, Kenji Yamada, Koji Nomura, and Takeshi Hosokawa, “Multipurpose Practical Sensor Network: S-wire” journal FUJITSU, May 2006 (Vol. 57, No. 3), pp. 285-290